1. Field of the Invention
This invention relates to a programmable logic device having a memory circuit for storing circuit configuration data and a logic gate block for realizing a logic circuit desired by a user on the basis of circuit configuration data written into the memory circuit, and more particularly to a programmable logic device having an improved degree of integration.
2. Description of the Prior Art
There has heretofore been widely used a programmable logic device (hereinafter referred to as a "PLD") comprising an integrated circuit having a construction in which an arbitrary logic circuit can be realized. The PLD of various types are provided for the users.
As means (devices) for selectively connecting arbitrary memory devices and logic devices of a logic gate block in the above-described PLD to one another to program a logic circuit, N-channel MOS transistors and P-channel MOS transistors are used as switching elements (transfer gates).
Furthermore, configuration of on-state or off-state of the switching elements for configuring a circuit of a logic circuit desired by the user in the logic gate block is determined in accordance with data of a memory circuit for storing circuit configuration data. Accordingly, the user can perform configuration of the logic circuit desired by the user by writing the circuit configuration data into this memory circuit for storing the circuit configuration data.
Applying the integrated circuits to electronic machines and components providing many advantages including rendering the electronic machines and components in a compact size and providing improved reliability of the electronic machines and components, lowered electric power consumption and so forth. Accordingly, many users utilize the above-described PLD to facilitate the circuit designing of the integrated circuits. Along with this trend, various electronic circuits of the electronic machines and components are selected as the targets for developing and providing the PLD of various types.
However, with the PLD in which the user can program an arbitrary logic circuit as described above, such problems are presented that a circuit configuration circuit used for programming the arbitrary logic circuit desired by the user must be provided in the PLD, thus lowering the degree of integration of the PLD.
In the above-described PLD, after the programming of the logic circuit desired by the user, the circuit configuration circuit used for programming of the logic circuit desired by the user becomes unnecessary. This arises as the logic circuit as programmed and configured by the user is sufficient for the finally required circuit in the integrated circuit.
That is, with the PLD, such a problem is presented that the degree of integration of the PLD as a whole is lowered by a layout space of the circuit configuration circuit used for programming and configuring the logic circuit desired by the user.
There is another type of PLD wherein a control circuit for programming the logic circuit desired by the user is not provided in the PLD and the control circuit for programming the logic circuit is connected to the PLD when the user programs the logic circuit desired.
However, with the above-described PLD, there are becoming necessary many wirings such as bit lines and word lines stretched around in the PLD for example, thus lowering the degree of integration of the PLD as a whole.
In recent years, there has been such tendency that, in the above-described PLD, the degree of integration is improved and the memory capacity of the memory circuit for storing the circuit configuration data provided therein is increased. Accordingly, such a problem cannot be neglected that the degree of integration is lowered by the wirings such as the bit lines and word lines used only for programming of the logic circuit desired by the user in the above-described PLD.
On the other hand, in IEEE 1990 CUSTOM INTEGRATED CIRCUITS CONFERENCE, pp. 31.2.1-31.2.7, there is suggested that the memory circuit for storing the circuit configuration data, i.e., a memory cell itself is utilized as a part of the logic circuit desired by the user after the configuration of the circuit.
However, no member other than the memory cell has been utilized.